<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            border: 1px solid red;
        }
        #d1{
            /*绝对定位，元素脱离文档流*/
            position: absolute;
            /*相对于窗口做位置偏移*/
            right: 0;
            bottom: 0;
        }
        #big{
            width: 200px;
            height: 200px;
            background-color: blue;
            margin: 100px 0 0 100px;

        }
        #big>div{
            width: 100px;
            height: 100px;
            background-color: green;
            margin: 50px 0 0 50px;
            position: relative;/*做参照物*/
        }
        #big>div>div{
             width: 50px;
            height: 50px;
            background-color: red;
            position: absolute;
            /*坐标偏移默认相对于窗口*/
            left: 0px;
            top:0px

        }
    </style>
</head>
<body>
    <div id="big">
        <div>
            <div></div>
        </div>
    </div>
    <div id="d1">div1</div>
    <div>div2</div>
    <div>div3</div>
</body>
</html>